This invention relates to a method for converting hydrogen and carbon monoxide to heavy hydrocarbons in a fixed bed reactor by a catalytic reaction where the catalyst pellet is designed so as to optimize the CO conversion and methane selectivity. Selectivity to methane is the percentage of the total CO moles converted.
A metal catalyst (e.g. cobalt or ruthenium) on a support (e.g., titania or silica) which may be promoted by different metals (e.g., rhenium, hafnium and others) are used for synthesis of heavy hydrocarbons from a mixture of carbon monoxide and hydrogen. The principal reaction may be expressed as: ##EQU1## where the distribution of the hydrocarbon products can be approximated by the Flory-Schultz expression. The fraction of oxygenates and olefins in the product is small.
An important consideration in the development of the hydrocarbon synthesis process is to minimize the production of light hydrocarbons (C.sub.1 -C.sub.4), especially of methane. The fraction of methane in the product exceeds that predicted by the Flory-Schultz distribution.
Another important consideration is to maximize the productivity, defined as the number of CO moles converted per unit time and reactor volume, so as to minimize the volume of the reactor in which the reaction is carried out.
Both considerations have been met with available catalyst powder of the size 80-140 mesh (approximately 0.15 mm in diameter). However, additional factors should be considered in the design of a fixed bed reactor; namely, the pressure drop in the reactor, and the removal of the heat generated by the reaction.
These require the design of catalyst pellets which retain the properties of the powder catalyst (80-140 mesh) but are larger in size (&gt;1.0 mm). However, since the reactants have to diffuse through liquid-filled pores, the longer diffusion path may create concentration gradients within the pellet. Such gradients alter the hydrogen to carbon monoxide ratio in the pellet due to the lower diffusivity of the latter. As a result the selectivity to methane, which depends on this ratio, increases considerably. Furthermore, since the rate of reaction depends on the concentration of the two reactants, the productivity is smaller in a pellet than in powder.
Because the pellets have to be used in a fixed bed reactor, the design of the catalyst pellet has to be directed toward minimizing the methane. selectivity and maximizing the productivity, where productivity is defined as the number of CO moles converted per unit time and reactor volume. The catalyst of the present invention is designed to achieve this purpose.